The present invention relates to cardiac pacers and other tissue stimulating devices, and more particularly to the calibration and testing of such devices to improve recognition of a desired result such as capture and to minimize the stimulation energy required to achieve the desired result.
Cardiac pacing devices are implanted in the body to deliver electrical stimulation pulses from a pulse generator to myocardial tissue, usually at the ventricle or the atrium. The function of each pulse is to cause a depolarization of the myocardial tissue at and near the point of pulse delivery, resulting in a ventricular or atrial contraction, i.e. a heart beat. This result is frequently referred to as "capture".
The energy of the stimulation pulse is a function of pulse amplitude and pulse duration. While the primary requirement of each stimulation pulse is an energy level sufficient to achieve capture, it also is advantageous to avoid excessive energy levels, to increase the life of the pulse generator battery. Thus, the goal is to stimulate the tissue with pulses near the minimum requirement for capture, i.e. the capture threshold, while providing a margin for safety.
The difficulty in achieving this goal arises in large part due to variations in the stimulation threshold. Not only does the threshold vary over different patients, it also may vary with time in each patient. More particularly, low thresholds are observed immediately after implant. Tissue inflammation at and near the stimulation electrode considerably increases the threshold during the first several weeks after implant. Over the longer term, reduction in the inflammation lowers the threshold, although the chronic threshold remains above the initial level at implant. Changes in an individual's activity, e.g. vigorous exercise, can cause short term changes in the stimulation threshold. Stimulation thresholds can be influenced by drugs such as catecholamines, beta-blockers, cardiosteriods and antiarrhythmic drugs.
The prior art includes examples of devices intended to change tissue stimulation levels to accommodate changes in stimulation thresholds. These devices typically incorporate an autocapture feature, e.g. a controlled varying of stimulation pulse levels in combination with a sensing of the cardiac response. In ventricle pacing, depolarization of the ventricle evokes an R wave (also known as a QRS complex) which can be sensed and processed to verify depolarization. Sensing circuitry can be incorporated into the pacing device. For example, see U.S. Pat. No. 5,330,512 (Hauck, et al). Other parameters can be sensed to confirm adequate stimulation levels, including oxygen concentration in the blood as in U.S. Pat. No. 5,176,138 (Thacker), and changes in fluid pressure in the heart as in U.S. Pat. No. 5,320,643 (Roline).
In comparing the above techniques, R wave sensing has the best potential for unambiguously indicating the presence or absence of an evoked response, i.e. capture verification. However, when the pacing device is used to sense the evoked response, lead polarization from the tissue stimulating pulse diminishes the ability to sense an evoked response and also can cause a false positive indication of capture. One attempt to address this problem is disclosed in U.S. Pat. No. 5,350,410 (Kleks, et al). Kleks teaches generating two stimulation pulses separated by a time less than the natural refractory period. The sensed response to the first pulse is assumed to include an evoked response and a lead polarization signal, while the second sensed response is assumed to include only the lead polarization Pulses at several levels are tested with several detection sensitivity levels to provide "polarization templates". To be judged an evoked response, a given signal must vary sufficiently from the polarization template. This approach, while perhaps diminishing the negative impact of lead polarization, does not address the variations in R waves and other evoked signals among different patients and over time in connection with a given individual. It also does not address the risk that the reliability of a previously designated response signal parameter may diminish, due to interference from physiological or extrinsic noise, or due to a physiological change.
Therefore, it is an object of the present invention to provide a process to verify capture or another desired response, that involves assessing the reliability of a chosen parameter of an evoked signal as a reliable indication of the response.
Another object is to provide a system, operable with a device that delivers stimulation pulses and senses evoked signals, for accumulating multiple measurements of a sensed signal characteristic to more reliably assess the utility of that characteristic in confirming capture.
A further object is to provide a device capable of sensing two or more characteristics of evoked signals and comparing the characteristics as to their efficacy in confirming capture or another desired result.
Another object is to provide a system which, at predetermined intervals, automatically and adaptively generates measurements used to demarcate signals or calculated parameters associated with capture and non-capture, respectively.
Yet another object is to provide a system and process associated with an implanted pacer or other stimulation device, for more effectively adjusting the level of stimulation pulses to track changes in a patient's stimulation threshold.